THIS INVENTION relates, broadly, to the production of propylene. More particularly, the invention relates to a process for the production of propylene, suitable for producing propylene from both pure and mixed hydrocarbon feedstocks.
From EP-A-O832867 a process for producing propylene is known.
According to the invention, broadly, there is provided a process for the production of propylene by butene metathesis which comprises contacting a feedstock, which contains butene selected from the group consisting of 1-butene, 2-butene and mixtures thereof, with a catalyst which includes a catalytic amount of at least one metal oxide selected from the group consisting of the oxides of the transition metals, under reaction conditions which include a temperature of 300-600xc2x0 C. and a pressure of 1-20 atmospheres, to convert the butene to propylene, thereby to produce a reaction product containing propylene.
In this specification any reference to 1-butene or 2-butene includes all the isomers thereof.
Conveniently, the temperature is 500-600xc2x0 C., eg about 550xc2x0 C. and the pressure may be about 1 atmosphere.
While the butene (which can also be referred to as butylene) may be in the form of a relatively pure butene feedstock, containing substantially only butene, it may instead form part of a mixed feedstock including, in addition to the butene, a plurality of other hydrocarbons. Thus, the feedstock may be a mixed hydrocarbon feedstock including, in addition to said butene, a plurality of other hydrocarbons.
In the latter case the feedstock may be a condensate, being for example a Fisher-Tropsch process-derived feedstock which is a condensate derived by reacting a synthesis gas mixture including carbon monoxide and hydrogen, in a selected CD:H2 ratio, over a Fisher-Tropsch catalyst which may be a cobalt-based catalyst, an iron-based catalyst, a catalyst based on both iron and cobalt, or a mixture of such catalyst. The feedstock or condensate derived from the Fisher-Tropsch process will typically not be subjected to cracking at any stage, and it may include, in addition to butenes such as the isomers 1-butene and 2-butene, one or more other alkenes or olefins, such as ethylene, propylene and various pentenes, hexanes and C7- or higher alkenes. In particular, the feedstock may be produced by a Fisher-Tropsch reaction at a temperature of 280-370xc2x0 C., preferably 300-340xc2x0 C., eg 320xc2x0 C., in a circulating fluidized-bed reactor or in a fixed-bed reactor, or in a fixed fluidised bed reactor at a pressure of 10-40 atmospheres (1 013 250-4 053 000 N/m2), preferably 15-25 atmosphere (1 519 875-2 533 125 N/m2), eg 20 atmospheres (2 026 500 N/m2), to produce a hydrocarbon product containing a range of hydrocarbon compounds with carbon numbers of C1-C38 or more, including olefinic hydrocarbon compounds.
In other words, more specifically, the feedstock may be a Fisher-Tropsch process-derived feedstock condensate derived by passing a synthesis gas mixture including carbon monoxide and hydrogen over a Fisher-Tropsch catalyst, the catalyst being selected from the group consisting of cobalt-based catalysts, iron-based catalysts, catalysts based on both iron and cobalt, and mixtures thereof; and the feedstock preferably contains, in addition to said butene, another alkene selected from the group consisting of ethylene, propylene, pentene, hexene, heptene, alkenes with more than 7 carbon atoms, and mixtures thereof. In this regard, the references to pentene, hexene, heptene and alkenes with more than 7 carbon atoms are references to all the isomers thereof. Preferably, the feedstock is a condensate derived from a high temperature Fisher-Tropsch process having a reaction temperature of 280-370xc2x0 C. and a reaction pressure of 10-40 atmospheres (1 013 250-4 053 000 N/m2), hydrocarbon compounds of the feedstock including, in addition to olefinic hydrocarbons, also methane and other hydrocarbons, at least some of the hydrocarbons having carbon numbers higher than 35.
When a Fischer-Tropsch-derived feedstock is used as described above, it may be fractionated prior to the contacting thereof with the transition metal oxide catalyst, to obtain a fraction, for use as the feedstock in the reaction, containing an increased proportion of desired alkenes. The fractionation may be by distillation, for example so-called short path distillation at a temperature of 150-400xc2x0 C. and at a pressure of 0.02-2.0 atmospheresxc3x9710xe2x88x923 (2.0265-202.65 N/m2), preferably 200-350xc2x0 C. and 0.07-1.5 atmospheresxc3x9710xe2x88x923 (7.09276-151.9875 N/m2), to obtain a fractionated feedstock having various proportions of C2 olefins, C3 olefins, xcex1-olefins, branched olefins and a small proportion of internal olefins, the fractionated feedstock being used for the production of propylene according to the present invention. In particular, the feedstock may be an uncracked feedstock which has been subjected to fractionation to increase the proportion of olefins therein.
Instead, a feedstock may be used which is a condensate stream from a naphtha cracking process or from a gas cracking process, including streams of Raffinate I, Raffinate II and/or Raffinate III.
By Raffinate I is meant the residue which is obtained when a C4 stream from a naphtha cracking process or from a gas cracking process (the C4 stream typically containing, as its chief components, n-butene, 1-butene, 2-butene, isobutene and 1,3-butadiene, and optionally some isobutane and said chief components together forming up to 99% or more of the C4 stream) is subjected to the removal of 1,3-butadiene therefrom. Thus removal may be by extractive distillation with an aprotic solvent such as acetonitrile, N-methylpyrrolidone or N,N-dimethylformamide, any remaining butadiene optionally being removed from the residue by an after-treatment such as selective hydrogenation, the residue being Raffinate I and containing, in substance, 1-butene, 2-butene and isobutene.
By Raffinate II in turn is meant a mixture of 1-butene and 2-butene which remains when Raffinate I has the isobutene separated therefrom by hydrogenation to tert-butanol in the presence of sulphuric acid; by reaction of the Raffinate I with methanol to synthesize methyl tert-butyl ether; or by oligomerization or polymerization of the isobutene.
Raffinate III in turn is meant what is obtained when the 1-butene in Raffinate II is separated therefrom by fractionation, extractive distillation or molecular sieve absorption, the residue of cis 2-butene and trans 2-butene remaining being Raffinate III. Raffinates I, II and III are described in more detail in Industrial Organic Chemicals, by Harold A. Wittcoff and Bryan G. Reuben, a Wiley Interscience Publication, published by John Wiley and Sons. Inc., New York, N.Y., U.S.A. (1996).
Another feedstock which may be used in a product obtained from dehydrogenation of paraffins, i.e. from dehydrogenation of alkanes, to yield olefins, i.e. alkenes, such products containing butenes, pentenes and higher olefins. Accordingly, the feedstock may be selected from Raffinate I, Raffinate II, Raffinate III and mixtures thereof; or it may be a product derived from the dehydrogenation of alkanes.
It is a feature of the invention that products are formed which are predominantly in the C2-C8 product range, with a propylene:ethylene mass ratio of up to 3:1, depending on the process conditions and catalyst used. The invention has been found to be capable of producing more propylene than ethylene from either a feedstock which consists of pure C4 olefins (butenes), for example C4 xcex1-olefins, or from a feedstock which is a condensate stream which includes such C4 olefins.
The transition metal oxide catalyst used for the catalytic conversion of butene to propylene is preferably selected from oxides of molybdenum, or of rhenium or of tungsten, or of mixtures of any two or more of tungsten, rhenium and molybdenium. This catalyst may be a homogeneous (unsupported) catalyst or a heterogeneous (supported) catalyst. Conveniently the catalyst is supported and any convenient support may be used, provided it neither interferes with nor inhibits the production of propylene from butene according to the invention. Suitable supports include those based on or including ceramic supports such as silica, alumina, titania, zirconia or mixtures thereof, with silica being preferred. The catalyst can be attached to its support in any convenient fashion, such as those known in the art, in particular by sublimation or by wet impregnation. The transition metal oxide constituent of the catalyst may amounts to 1-30% by mass of the total catalyst mass (transition metal and support together), preferably 6-20% thereof. If desired, the catalyst mass may include one or more promoters such as phosphates, borates or magnesium oxide, which increase selectivity towards propylene. Instead, the catalyst mass may include one or more promoters which are alkali metals, such as lithium, sodium, potassium and cesium, which decrease selectivity towards propylene. In particular the catalyst may be a WO3-based (tungsten oxide-based) catalyst containing Cs (cesium) as a promoter.
In other words, the transition metal oxide catalyst used for the conversion of butene to propylene may be selected from the group consisting of the oxides of molybdenum, the oxides of rhenium, the oxides of tungsten, and mixtures thereof; and the transition metal oxide catalyst for the conversion of butene to propylene may form part of a catalyst mass in which the transition metal oxide is supported on a solid ceramic support of a material selected from the group consisting of silica, alumina, titania, zirconia and mixtures thereof. In this case the transition metal oxide may form 1-30% by mass of the total heterogeneous catalyst mass. In particular, the catalyst of the heterogeneous catalyst mass may consist essentially of tungsten oxide, being supported on silica, the catalyst mass containing a promotor selected from the group consisting of cesium (to decrease selectivity towards propylene) and phosphates (to increase selectivity towards propylene).
Preferably the catalyst has as high a proportion of acid sites thereon as practicable. Thus, tests conducted by the Applicant have shown that carrying out the process of the present invention with a catalyst having a relatively high proportion of acid sites favours the production, from butene, of propylene (and pentenes) over ethylene (and hexanes). Acidity of the catalyst can be enhanced by treating the catalyst with organic acids or inorganic acids, or by impregnation thereof with cations such as those of phosphates or borates. Acidity of the catalyst can, conversely, be reduced by blocking acid sites thereon, by means of alkaline earth metals, the presence of which alkaline earth metals can improve the selectivity of the reaction in favour of ethylene and hexenes.
The reaction product may be separated into its constituent parts by conventional separation techniques, for example into an ethylene-containing fraction, a propylene-containing fraction, a C4-containing fraction and a fraction including C5- and higher hydrocarbons, with the C4 fraction optionally being recycled to the reaction. Thus, unreacted butene may be separated from the reaction product, the separated unreacted butene being converted to propylene by recycling it and contacting it, together with the feedstock, with the transition metal oxide catalyst; and C5- and higher hydrocarbons may be separated from the reaction product, the separated C5- and higher hydrocarbons then being converted to propylene by recycling them and contacting them, together with the feedstock, with the transition metal oxide catalyst. This recycling can act to increase the yield of propylene. The invention has been shown to be capable, from a pure 1-butene feedstock, of obtaining a product having a product spectrum as set forth in the following table, Table 1:
The invention will now be described, by way of non-limiting illustration, with reference to the following illustrative Examples: